1. Field of the Invention
The present invention relates to a spherical, ellipsoidal or plate-like vaterite type calcium carbonate almost free from secondary aggregation and well dispersible, its manufacturing method and a method of controlling the growth of particles and shape of vaterite-type calcium carbonate in which calcium carbonate is used as matrix and the particle size and shape of the matrix are varied so as to prepare vaterite type calcium carbonate different from the matrix and improved in dispersibility compared with the matrix vaterite type calcium carbonate different therefrom in particle size and shape.
2. Description of the Prior Art
As industrial manufacturing method of synthesized calcium carbonate, the carbon dioxide method is widely adopted. This carbon dioxide method comprises steps of first obtaining unslaked lime (calcium oxide) by calcinating limestone occurring in nature, letting this unslaked lime react with water to form milk of lime (aqueous suspension of calcium hydroxide) and then letting carbon dioxide gas resulting from calcination of limestone pass through and react with this milk of lime to thereby obtain calcium carbonate.
The synthetic calcium carbonate manufactured by this carbon dioxide gas method is used in large quantities as filler or pigment for rubber, plastics, paper, paints et cetera according to the size of the primary particles.
The synthetic calcium carbonate for these uses is surface-treated with various inorganic or organic treating agents according to the intended use for further improvement of its physical properties when mixed in such products.
The synthetic calcium carbonate manufactured by this carbon dioxide gas method, however, is essentially extremely high in inter-primary particle cohesive power and large numbers of primary particles aggregate to a large secondary particle (coarse aggregate of primary particles) and the slurry of these secondary particles is impossible to disperse as primary particles even by prolonged powerful stirring.
When synthetic calcium carbonate containing a large number of aggregates of primary particles is used a as filler or pigment for rubber, plastics, paper, paints or the like, the secondary particles shows a behavior as if they were primary particles, hence no good physical properties are obtained, the consequence being poor dispersibility, lowered physical strength, lowered luster and aggravated fluidity, the mixing effect essentially as with primary particles being thus unattainable.
Even if such surface treatment by the use of inorganic or organic treating agents should be applied to synthetic calcium carbonate containing a large number of aggregates, it means only treatment of the surface of secondary particles and no sufficient treatment can be hoped for.
Although many methods have been reported to date for dispersion of such aggregates of primary particles, generally adopted is the method of powerfully grinding and breaking them using a ball mill, sand grinder mill or the like. Since, however, this method is a milling mode of grinding, dispersion of aggregates thereby is accompanied by breaking of primary particles, this resulting in coexisting of particles extremely unstable in surface conditions and smaller in size than the desired primary particles and aggregated secondary particles dispersed insufficiently and can hardly be preferable, for the particle size distribution is quite broad.
A wet grinder such as sand grinder mill usually uses extremely small glass beads as grinding media. Since, however, the surface of such glass beads is ground and broken in the process of grinding and breaking calcium carbonate, the calcium carbonate having undergone dispersion has mixed therein a large number of coarse pieces of glass of 20 .mu.m or so, and it is not advisable to use such wet-grinding method for dispersion of calcium carbonate for use as filler for films as thin as 15 .mu.m or so.
Calcium carbonate has as polymorphisms, calcite type crystals of hexagonal system, aragonite crystal of rhombic system and vaterite type crystals of pseudohexagonal system. Of all these, however, now being industrially manufactured and being applied to various uses are, for the most part, cubic or spindle shaped calcite type crystals and needle or columnar aragonite crystals.
In contrast thereto, vaterite type calcium carbonate is rather better in dispersibility than the other two crystal forms and also because of freedom from large coarse aggregates it is expected to be effective for improvement of coating property as well as of filling performance and also for improvement of the product's physical strength, luster, whiteness and/or printing property.
From the above viewpoint, various methods have been studied for industrial manufacture of vaterite type calcium carbonate.
For example, in Japanese Laid-open Patent Publication No. 90822/'85 there is disclosed a method in which vaterite type calcium carbonate is obtained by introducing carbon dioxide gas into aqueous suspension of calcium hydroxide including magnesium compounds and then adding alkali polyphosphate or alkaline metal salt thereof when a certain carbonation ratio has been reached, and in Japanese Laid-open Patent Publication No. 150397/'79 disclosed is a method of adding ammonia to the calcium chloride/calcium hydrogen carbonate reaction system so that the slurry's pH value reaches 6.8 upon completion of the reaction.
Both of these methods, however, are not only more complicated than the conventional manufacturing method for cubic or spindle shaped calcium carbonate but their use also makes it difficult to control the size of vaterite type calcium carbonate particles so that the primary particles of the obtained vaterite type calcium carbonate are non-uniform in particle size, and are also lacking in dispersibility.
Lately, there have been proposed various methods for manufacturing vaterite type calcium carbonate in which the same is manufactured by carbonating calcium hydroxide contained in some organic solvent.
For example, in Comparative Example 1 of Japanese Laid-open Patent Publication No. 64527/'84 there is disclosed a method of manufacturing calcium carbonate of vaterite type by blowing carbon dioxide gas into a mixed solution of a calcium hydroxide aqueous slurry and methanol, and in Japanese Laid-open Patent Publication No. 77622/'86 described is a method of manufacturing noncrystalline or crystal calcium carbonate of vaterite type or the like by blowing carbon dioxide gas into a suspension liquid system of calcium hydroxide-water-alcohol.
Although it is possible to obtain vaterite type calcium carbonate at a high yield, arbitrary control of the particle size and shape thereof is infeasible, a further defect being that stable manufacture of monodisperse spherical, ellipsoidal or plate-like vaterite type calcium carbonate is also infeasible.
Lately, especially for uses in highly advanced technical fields there are required, for development of industrial products of still higher functionality, monodisperse inorganic particles of controlled size and shape a high degree of dispersibility.
For example, with polyester film used for the manufacture of magnetic tape such as audio tape, video tape et cetera, its slipperiness and anti-shaving property are important factors on which depend the workability in the film manufacturing process as well as in the process for manufacturing various secondary products and also the quality of the individual products attainable. If the slipperiness and anti-shaving properties are insufficient or unsatisfactory, when, for example, a magnetic layer is formed by coating on the surface of polyester film and the film so coated is used as magnetic tape, the friction between the coating roll and the film surface as the magnetic layer is formed by coating is quite marked as well as the wear in film surface and in extreme cases it results in formation of creases, abrasions and the like. Even after slitting the film with a coated layer of magnetic material and finishing as audio tape, video tape or tape for computer, marked wear results between such tapes and guides, playback heads and the like as the tape is pulled out of a reel, cassette or the like or as it is wound up, this causing generation of abrasion, strain and the like and separation of chalk-like substance due to shaving of polyester film surface. And it often results in the so-called drop-out of magnetic recording signal.
For lowering a polyester film's friction coefficient there have, hitherto, been proposed many methods in which fine particles are incorporated in the polyester to impart a minute and proper degree of roughness to the surface of a molded article for properly controlling or improving the surface slipperiness thereof. So far, however, there was much to be desired about the compatibility between such fine particles and the polyester, nor and the film's transparency and its wear resistance are not satisfactory. To further explain this method, there have hitherto been proposed many methods for improving the surface properties of polyester as follows:
(1) a method in which a part or whole of the catalysts and the like used in synthesis of the polyester are caused to separate in the reaction process (internal particles separation system).
(2) a method in which fine particles such as calcium carbonate and silicium dioxide are added during or after polymerization (external particles adding system).
With such particles for imparting a proper degree of roughness to the surface of polyester film it is generally accepted that the larger their size, the higher the effect of improving slipperiness. With magnetic tapes, especially such high precision tapes as video tape, large particle size itself may possibly cause defects such as drop-out and for this reason the degree of roughness in the film surface is required to be as low as possible, although these requirements are apparently contradictory.
The (1) internal particles separation system is a method in which the particles are metallic salts of polyester component or the like, hence their compatibility with polyester is somewhat satisfactory but, since, at the same time, it is a method in which the particles are caused to form in the course of reaction, it is difficult to control the quantity as well as size of particles and prevent formation of coarse particles.
The (2) system is a method in which the quantity and size of particles to be added are properly selected and the slipperiness attainable is further improved through addition of fine particles with coarse particles eliminated by classification or the like.
Since the compatibility between inorganic fine particles and polyester as organic component is insufficient, exfoliation takes place in the boundary between inorganic particles and polyester, this resulting in formation of voids. If such voids should exist in polyester, inorganic particles are apt to release from polyester film due to damage to polyester film through contact between polyester films as well as between polyester film and other substrate, this possibly resulting in chalking of film for magnetic tape and the phenomenon called drop-out.
Although for manufacture of polyester film the methods (1) and (2) are currently being used as alternatives but the method (2) has been gradually adding importance because of its greater ease of selection of particle size as well as of reproducibility of quality. Since the inorganic particles used in the method (2) has insufficient compatibility with polyester, however, chalking due to release of particles from polyester film caused by damage to polyester film is likely to occur. In order to preclude this phenomenon, chemical approach is being made by development of and research about effective surface treating agents for inorganic fine particles, while physical approach is being made by development of and research about inorganic fine particles in such shapes difficult to be released from polyester film. As to the shape of inorganic fine particles used in the method (2), ellipsoidal or plate-like particles are taken to be better than spherical particles from viewpoint of difficulty to release from polyester film but spherical particles are said to be ideal from viewpoint of film's
slipperiness, which is another important physical property.
As plate-like inorganic particles for use in the method (2), kaolin clay prepared by elimination of coarse particles by the use of a special classification technique is mainly used. Since the kaolin used in this field is one refined from natural kaolin, the primary particles are extremely non-uniform in particle size as well as shape. Since it is taken that complete elimination of coarse particles is not feasible even by repeated classification using advanced classification technique and for this reason kaolin has no possibility as anti-blocking agent of high-grade polyester film, development of plate-like particles uniform in particle size, substantially not containing aggregate particles and well dispersible has long been waited for.
As to the plate-like particles of calcium carbonate, research has hitherto been made from various angles. For example, there have been proposed a method of first synthesizing calcium hydroxide and then carbonating it under heating to obtain calcium carbonate with the particle shape of calcium hydroxide, and a method of adding calcium hydrogen carbonate to water under controlled heating. The platelike calcium carbonate obtained by those methods is extremely broad in particle size distribution and there is something to be desired about its dispersibility and the latter manufacturing method has a defect of being difficult to control the particle size of plate-like calcium carbonate in its commercial manufacture.
Further, research and development of plate-like basic calcium carbonate represented by the composition formula CaCO.sub.3. xCa(OH).sub.2. yH.sub.2 O as similar to plate-like calcium carbonate are being made in various fields and typical of such manufacturing method are described in Japanese Laid-open Patent Publication No. 219715/'86, Japanese Laid-open Patent Publication No. 113718/'87 et cetera.
Although basic calcium carbonate of this kind has CaCO.sub.3 in its composition formula, it is a substance entirely different from calcium carbonate. It can generally exist stably only in a highly alkaline range and, with its pH being much higher than that of ordinary calcium carbonate, it is usable only in a narrow field, which is a great defect.
As spherical inorganic particles for use in the conventional method (2), a monodisperse spherical silica obtainable by hydrolysis of alkoxysilane and, condensation reaction et cetera has been developed and, since it is uniform in particle size, good in dispersibility in ethylene glycol as well as polyester, its possibility as anti-blocking agent for video tape with its especially high requirement about quality is being studied. This spherical silica is difficult to provide economically due to the very high cost of alkoxysilane as its material, and requires a very long time for reaction due to slow progress of hydrolytic reaction. It is apt to have its spherical particles released from polyester film due to damage to polyester film, for its compatibility with polyester is insufficient compared with other inorganic particles externally added to polyester, this likely resulting in the risk of chalking and drop-out when it is used for manufacture of film for magnetic tapes. The spherical silica has another fatal defect of being high, not less than 6, in Mohs hardness and the spherical silica released from polyester film has a risk of damaging the surface of playback head of video tape recorder.
Hence, with the inorganic particles for use in the manufacture of such polyester films required is establishment of morphological control techniques for delicate control of particle size and shape such as particle's shape controlling technique, for example, modifying ellipsoidal particles to be closer to real sphere, ellipsoidal particles to spherical particles, spherical particles to plate-like particles, or plate-like particles to those increased in thickness, and particle size control technique for increasing particle size with the solid geometrical similarity in shape maintained, to say nothing of requirement about uniformity of particles and about high dispersibility. Another essential requirement is that Mohs hardness of such inorganic particles be relatively low, being 3 or so.
Since Mohs hardness of calcium carbonate is approximately 3, being thus relatively low among known inorganic particles, development of calcium carbonate which is at least equivalent to the aforementioned spherical silica as externally added inorganic particles in uniformity of particles and dispersibility and with which delicate morphological control of particle's shape and size et cetera has long been awaited.
As to calcite type calcium carbonate and aragonite type calcium carbonate as polymorphisms of vaterite type calcium carbonate, various methods have been reported to date about techniques of controlling particle size with the shape of cubic or needle-shaped particles maintained, if the poor uniformity of shape of particles and poor dispersibility are disregarded. As to vaterite type calcium carbonate, especially that good in dispersibility and uniformity of particles, no report has been made to date even about particle size control techniques, to say nothing of particle's shape control techniques.